How to store php sessions using memcached and unix socket

I am running Arch linux with systemd, nginx, and php with php-fpm. I am trying (and failing) to configure memcached to store sessions using a unix socket. I have memcached installed and active, however, I am unable to disable networking. I added the following line to /etc/conf.d/memcached

when I run php script with sessions php error log shows (not surprisingly):

[19-Jan-2013 16:46:45 America/Los_Angeles] PHP Warning: Unknown: Failed to write session data (memcached). Please verify that the current setting of session.save_path is correct (unix:/tmp/memcached.sock) in Unknown on line 0

I also installed the php-memcached package but I don't what it does or how to get it going. I uncommented the following line in /etc/php/conf.d/memcached.ini:

extension=memcached.so

but that didn't change anything.

Any insights or suggestions would be greatly appreciated.

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